Old boy in tight trio constant like automaton (7)
I believe the answer is:
robotic
'like automaton' is the definition.
(I know this)
'old boy in tight trio constant' is the wordplay.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'in' is an insertion indicator.
'tight' indicates an anagram (tight can mean drunk).
'constant' becomes 'c' (mathematical abbreviation).
'trio' is an anagram of 'roti'.
'ob' inserted into 'roti' is 'roboti'.
'roboti'+'c'='ROBOTIC'
(Other definitions for robotic that I've seen before include "Mechanistic" , "Using automatic" , "acting without thinking?" , "Like a machine man" , "Stiff, unemotional" .)